1. Field of the Invention
The present field of the invention relates to membrane switches, and more particularly to a method for manufacturing membrane switches that are illuminated using electroluminescent lamps.
2. Description of the Prior Art
Present membrane switches are typically made from flexible plastic insulators that contain two layers of opposing electrically conductive surfaces isolated from one another by an air gap such that, when one surface is mechanically deformed by applied pressure, that deformed surface makes mechanical contact against the opposing stationary surface and completes an electrical current path between them. This current path may carry either signal or power electrical charge, or both. By positioning an insulating mask between these two surfaces, effective mechanical isolation ensures that unwanted electrical contact is avoided. Adding illumination to such membrane switches can create both complicated and bulky assemblies tat are unsuitable for many electronics product applications. Illuminated membrane switch assemblies made using this method contain three or more individual layers of electrically conductive and isolating materials that require precise alignment for their successful application.
An alternative construction consists of a rigid circuit board having on its upper surface a pair of electrical switch contacts. Positioned above this surface is an isolating mask layer that is typically a plastic film with openings positioned in alignment with the contact pairs. Above that is placed a second plastic film with a deformable electrical shunt surface oppositely positioned in alignment with the isolation mask's opening and the printed circuit board's switch contact pairs. When this outermost shunt layer is mechanically deformed by pressure, the shunt is driven past the isolating mask layer opening such that the shunt may then make contact to the printed circuit board's switch contacts, thus creating a current path. Illuminating this switch constructions may take the form of an overlaying elastomeric actuating structure that is edge-lit illuminated by externally mounted lamps or alternatively via light emitting diodes (LED's). Application of an additional layer of electroluminescent lamp construction may also be used to provide illumination to the elastomeric structure. Such constructions typically require an additional rigid framework to keep the various layers in alignment.
An alternative to this s construction is to form the elastomeric actuating structure into an integrated system that begins with a positioning flange that rests on top of the printed circuit board and surrounds the switch contact pair. Projecting from this flange structure is an elastomeric spring member that then supports an actuating key. In the open gap formed by this structure, a typically cylindrical shaped protrusion extends down from the actuating key and is supported above the switch contacts. The end of this protrusion may alternatively be coated with a conductive surface to provide the electrical shunting effect, or a “pill” of conductive elastomer is attached to the protrusion to provide this function. Thus, the actuating key may be pressed, allowing the shunting surface of the protruding conductor to mechanically contact the switch contacts below to from an electrical current path between them. If an additional insulating layer, constructed with electroluminescent lamp elements that surround an opening in the insulation corresponding to the location of the shunting protrusion of the elastomeric actuating structure, is placed between the elastomeric actuating structure and the surface of the switch bearing side of a printed circuit board, a ring of illumination surrounds the actuating key. Additionally, a rigid framework must also be provided to keep the surfaces and structures in alignment.
In the above alternative methods, only signal level electrical charge may be switched by key actuation. Additionally, these structures are also bulky, and require great care in their design and manufacture in order to make them successful for many electrical and electronic applications.
To provide a pleasing tactile “snap” to the above constructions, a layer of formed metal foil shapes may also be applied to replace the shunt layer. These shapes are typically convey on their outer surface and concave on their interior surface. By placing the formed metal foil shapes above the isolating mask layer opening, opposite a switch contact pair, applied mechanical pressure causes the shapes to temporarily invert, thus making contact between the switch contacts. This method allows both signal and power electrical charges to be passed between switch pairs. As this construction also requires individual layers to be assembled, including illuminated actuating elastomeric structures and frames, a bulky and complex assembly results.
Application of electroluminescent lamp as an illumination scheme to the above methodologies provides a thinner structure, however there are still numerous individual layers and actuators to be applied and aligned to complete an illuminated membrane switch assembly. An example of this process is referenced in U.S. Pat. No. 5,680,160 (the '160 patent), wherein LaPointe describes such an application consisting of screen-printed illumination and electrical contacts arranged in a pattern such as might be used for a map as a teaching tool in geography. However, this method only provides illumination during switch contact, and is also limited in the amount of electrical current the switch contacts may carry. The use of conductive inks as switch elements also severely limits their useful life cycle. Additionally, this method does not provide electrical circuit separation between the switch portion and the illumination circuit portion without introducing an additional switch contact and shunt set with attendant construction and isolation layers. Thus, high voltage alternating current may add electrical interference to the switch circuit. As the switch circuit may also make contact for voltage sensitive semiconductor devices, this lack of isolating circuits may cause both electrical interference to, and failure of such devices.
In U.S. Pat. No. 5,667,417, Stevenson teaches a method of producing low cost metal foil based electroluminescent lamps of potentially complex graphic pattern by using a precise indexing system that applies well known flexible circuit technology to a cost-effective continuous production process. Application of this process to the manufacture or illuminated membranes switches can result in switch assemblies that are both low-cost, plus electrically and mechanically superior to those described in the '160 patent.
Thus, there is a need for low profile illuminated membrane switch assemblies that provide all the elements of individually addressable illuminated areas, electrically separated switch and illumination circuitry, plus robust current carrying switch contacts and shunting means. Further, there is a need to provide such a low profile membrane switch assembly that may be made from a single flexible substrate material applied to an automated manufacturing system.